1. Field of the Invention
The present invention relates to a method of driving a liquid crystal display device for performing color display, more particularly to a method of driving a liquid crystal display device for displaying information corresponding to a picture of a composite video signal in color.
2. Description of the Related Art
There has been widely used a liquid crystal display device capable of performing a color display as a display of various information from apparatuses and instruments such as a liquid crystal color TV, a personal computer, etc.
A prior art basic driving method of such a liquid crystal display device will be explained with reference to FIG. 16.
A composite video signal CV is converted into RGB analog signals by a chrominance circuit 50 which are inputted to a driving circuit 51. The driving circuit 51 actuates a liquid crystal panel 52 based on the voltage levels of the RGB analog video signals to control the transmittance of each pixel corresponding to each color of red, green and blue on a liquid crystal panel so as to perform color display.
The composite video signal may be of the NTSC system, the PAL system or the SECAM system. The NTSC system will be described hereinafter.
FIG. 17 shows an example of a composite video signal. This signal is converted into RGB analog video signals having red, green and blue colors respectively by the chrominance circuit 50.
FIG. 18(a)-18(c) show the converted RGB analog video signals. In the figure, denoted at An is the maximum amplitude of each of the RGB analog video signals.
The driving circuit 51 controls the transmittance of each pixel corresponding to each color of red, green and blue on a liquid crystal panel 52 in response to the voltage levels of the inputted RGB analog video signals to faithfully reproduce the image of the RGB analog video signals thereon.
FIG. 19 shows the relationship between the RGB analog video signals and the transmittance of the liquid crystal panel. In the figure, the lateral axis represents the voltage levels of the RGB analog video signals while the vertical axis represents the transmittance TLCD of the liquid crystal panel. A T-V curve c represents the transmittance characteristic.
When the voltage level of the RGB analog video signal is as large as the maximum amplitude An, the transmittance of the liquid crystal panel becomes 100%, and when the voltage level thereof is 0, the transmittance also becomes 0%. A color picture is displayed based on the transmittance characteristic.
As mentioned above, in the prior art liquid crystal display device, there is only one transmittance characteristic as shown in FIG. 19 so that an image represented by a composite video signal is represented based on the same transmittance characteristic irrespective whether color or monochrome. Accordingly, the color reproducibility of a color picture displayed on the prior art liquid crystal display device is determined by the color demodulation performance of a chrominance circuit 50 in FIG. 16, and hence the demodulation performance of the chrominance circuit is regarded as important.
Whereupon, even if the chrominance circuit 50 having a high demodulation performance is employed, a picture actually displayed on the liquid crystal display device appears inferior to that displayed on a color CRT in color reproducibility. This is mainly caused by the NTSC broadcasting system. The NTSC system is conceived on the premise of employment of a CRT as an image receiver, and it skillfully employs many techniques utilizing characteristics of the CRT and of human visual processing and interpretation.
However, the CRT is different from the liquid crystal display device in a method of display. The NTSC system sometimes causes inconvenience in color reproducibility on the liquid crystal display device, which is different from the CRT in the method of display.
The CRT performs color display when an electronic gun emits an electronic beam to a fluorescent body to make the same emit light. Contrary to this, the liquid crystal display device is a light-receiving type display device of a light bulb system which controls the transmission of light emitted from a back light.
A cold-cathode tube is frequently employed as the back light, but a halogen lamp or a metal halide lamp is employed in a liquid crystal video projector and the like.
The light-receiving type liquid crystal display device employing the back light is short of brightness (luminance) compared with a light-emitting type CRT as matters stand. An opening ratio representing the ratio of area of the liquid crystal display device through which light is transmitted is about 30% in an active system. The 30% transmitted light is further absorbed by a deflecting plate and a color filter, and hence an actual amount of light transmitted therethrough is at most several percent of that emitted from the back light.
As mentioned above, the light utilization efficiency of the liquid crystal display device is inferior to that of the CRT so that a picture on the liquid crystal display device is much reduced in brightness compared with that on the CRT. Particularly, there is a problem of luminance shortage in the liquid crystal projector since a projection system is liable to be influenced by the brightness of the environment adding to insufficient brightness of the back light. Various inconveniences occur if the composite video signal of the NTSC system which premises the CRT for a display is displayed faithfully on such a liquid crystal panel which is inferior in brightness.
Firstly, in case of monochrome display having no color, the picture of the liquid crystal display device is less bright than that of the CRT but the former is reduced in brightness as a whole, a picture thereon can be seen sufficiently so long as it is bright to some extent. Moreover, in the case of the liquid crystal display device having the cold-cathode tube as the back light which has high color temperature, it gives a feeling close to that of the CRT.
Whereupon, a color display on the CRT is largely different from that of the liquid crystal display device in color reproducibility of the picture. If the same picture is displayed on the CRT and on the liquid crystal display device, the liquid crystal display device seems short in color saturation.
This is caused by the fact that for human beings color discrimination ability is reduced as luminance is lowered. Further, there occurs a Bezold-Bruecke phenomenon, that a color is recognized to be different in hue at a low luminance. Blue-green light seems to be more bluish and orange light seems to be more yellowish.
Such a phenomenon largely influences display quality. Since these are all caused by a shortage of luminance, such a phenomenon occurs remarkably in the projection type liquid crystal video projector rather than the liquid crystal display device which employs the cold-cathode tube as the back light. Particularly, in the case of a small 1-chip liquid crystal video projector employing a halogen lamp, it is remarkably short of luminance, which largely influences visual performance.
To cope with the aforementioned problem, there has been employed a method of enhancing color saturation using a chrominance circuit. This method is performed by separating a composite video signal CV into Y/C signal components in the chrominance circuit 50 as shown in FIG. 16, amplifying a C (color difference) signal component to increase the ratio of the C signal component relative to the Y (luminance) signal component thereby enhancing the color saturation.
Since most of the integrated-circuit chrominance circuits previously included therein a color adjusting circuit, this method could be relatively easily employed.
However, this method has a significant problem. That is, if the color adjustment is emphasized, a color noise is likely to occur due to cross-talk between color signals. Since the occurrence of the color noise remarkably deteriorates the display characteristic, there is a limit for enhancing the color saturation merely by the chrominance circuit.
The present invention solves the aforementioned problems and provides a liquid crystal display device capable of enhancing color saturation of a picture displayed in color on the liquid crystal display device to thereby improve the display quality without generating a color noise.